A number of techniques have been developed for characterizing the surface topography, voltage potential, and capacitance distribution of semiconductor devices. These techniques have been developed in response to advances in semiconductor technology, in which the dimensions of processed semiconductor devices are becoming ever smaller. This diminution in device scale renders both physical and electrical analyses more difficult to perform. Traditional methods of electrical measurement, such as direct mechanical probing, tend to become difficult or impossible to perform at such reduced scale.
Specifically, present-day very large scale integrated (VLSI) circuit technology demands accurate knowledge of the spatial extent in three dimensions of active impurity dopants which have been incorporated into the discrete device elements. The devices are predominantly either bipolar or metal oxide semiconductor field effect transistors (MOSFET's), diodes, or capacitors. A typical device occupies an area on the order of 10 .mu.m.sup.2. The active region of such a device, where most current flows, is engineered by incorporating dopants, for example arsenic, boron, or phosphorous, in a concentration range of 10.sup.15 to 10.sup.20 cm.sup.-3. It is necessary to control the variation, or profile, of impurity dopants to a spatial resolution of 100 nm (1 .mu.m equals 1,000 nm) or less for high yield in manufacture and for reliability of the circuitry in the field. Lack of precision related to the incorporation of impurity dopants can result in a proliferation of undesirable defects during later steps in the manufacturing process, less than adequate device performance, or even device failure. Such high precision in the characterization of dopant profiles on a microscopic scale is, clearly, highly desirable for efficient device design. In order to achieve predictability in device behavior, one must be able to measure accurately the dopant profiles and feed this information back into the design cycle. It has been impossible to achieve this high precision, except in one dimension, either in the design or manufacturing phases of VLSI components on the submicron scale. The need exists, therefore, for an instrument able to fulfill all of the above criteria for dopant density profiling in two and three dimensions.
Hence, efforts have been directed to devising electrical analysis instruments which are non-destructive, do not contact the sample, and exhibit improved spatial resolution. Included among recently developed electrical analysis instruments are microscopes based upon local interactions between a probe having an atomically sharp tip and a sample surface. Such interactions include electron tunneling, atomic force, magnetic force, as well as thermal, optical, and electrostatic coupling. Scanning probe microscopy refers generally to a class of high resolution techniques for studying surfaces at or near atomic resolution. Several different techniques which produce these results have been described in the prior art.
One of the first such techniques is scanning tunneling microscopy. In the scanning tunneling microscope (STM), a sharpened tip is maneuvered to, and held in electrical contract with (circa 0.1 to 1 nm above), a conducting sample surface and biased to produce a current between the tip and the surface. The tip is sufficiently close to the surface that there is an overlap between the electron clouds of the atom at the probe tip and of the nearest atom of the sample. When a small voltage is applied to the tip, electrons "tunnel" across the gap generating a small tunneling current. The strength of that current is very sensitive to the width of the gap. Piezoelectric controls are used to control the motion of the probe and to move it back-and-forth across the sample while maintaining a constant gap between its tip and the sample surface. The variations in voltage applied to maintain the probe properly positioned over the surface are electronically translated into an image of the surface topography. The tip and the sample in this technique must be electrically conductive to allow current flow between them, which limits the application of the technique. Another limitation is that the STM is sensitive only to the charge density at the surface of the sample.
The invention of STM led to the development of a family of new scanning probe microscopes, one of which is the atomic force microscope (AFM) which negates the need for a conducting sample. In its first implementation, the AFM relied upon the repulsive forces generated by the overlap of the electron cloud at the surface of the tip with electron clouds of surface atoms within the sample. A typical AFM microprobe tip is fabricated on the end of a mechanically compliant cantilever so that the axes of the cantilever and the tip are substantially perpendicular to one another. The process most widely used to form the cantilever-tip system involves masking and etching crystalline silicon, a process which is described in the prior art. The silicon cantilever is typically 100 .mu.m long and 1 or 2 .mu.m thick. The tip is usually tetrahedral in shape and 2 to 3 .mu.m both in height and along the base of the tetrahedron, which is integral with the cantilever, having been etched from it. The silicon tip senses the sample surface as it comes into close proximity (on the order of 3 nm) with the sample surface as the cantilever oscillates mechanically above the surface near the cantilever longitudinal resonance frequency (which is usually several tens of kHz).
This is because, as described in the prior art, the repulsive (van der waals) forces between the tip and surface of the sample cause deflections of the cantilever resulting in deviations from the cantilever mechanical resonance. These deviations are sensed as a voltage and fed back to a piezo-transducer which adjusts the position of the tip-cantilever relative to the sample surface so as to bring the cantilever back to its operating resonance. Thus, changes in the resonance of the tip-cantilever system provide a sensitive measure of the deflections of the cantilever, and hence of the forces between the sample surface and the tip. Although a number of methods have been used to measure the movement of the cantilever and tip combination, measurements have been made recently by optical methods including a laser beam. The reflected laser beam is detected and enables cantilever movements to be converted to imaging signals.
Another variant of the AFM with which the present invention can be used is the electrostatic-force microscope (EFM) where the vibrating, conductive probe is charged electrically by application of an electric potential difference to it relative to the sample surface. When the EFM is capacitively coupled, via a resonant tank circuit and lock-in amplifier as taught in the prior art, to the vertical axis (i.e., the z-axis) piezo-transducer which controls the tip height above the sample surface, the EFM is known as a scanning capacitance microscope (SCM). Furthermore, if the SCM piezo-transducer feedback voltage is dynamically adjusted to force the tip-to-sample potential difference to be zero, the SCM is operating as a scanning Kelvin probe microscope (SKPM) as is taught in the prior art. In this case, the measured quantity is referred to as Kelvin voltage or, equivalently, contact potential difference. Another form of AFM is a magnetic-force microscope (MFM). In an MFM, a magnetized nickel or iron probe is substituted for the tungsten or silicon needles used with other AFM's. When the vibrating probe is brought near a magnetic sample, the tip "feels" a magnetic force that changes its resonant frequency and hence its vibration amplitude. The MFM traces magnetic-field patterns emanating from the sample.
In summary, both SCM and SKPM have been shown in the prior art to be effective techniques for the measurement of engineered, impurity dopant profiles and fixed charges and for ensuring local electrical integrity for product quality in production, sub-micron VLSI devices. Quantitative two-dimensional dopant profiling for calibration of process simulators on a nanometer scale has been identified, in fact, as one of the significant needs of the semiconductor industry. The development of SCM as a profiling tool has been spurred by its inclusion in the Semiconductor Industry Association's National Technology Roadmap for Semiconductors. SCM and SKPM permit the simultaneous acquisition of both electrical and topographical data, as mandated by the Roadmap, as the tip-cantilever is scanned over the semiconductor sample surface. Improvement in efficiency, repeatability, and data quality of SCM and SKPM are therefore desirable in the art.
Ultrasmall tips etched on silicon cantilevers have become a standard transducer for scanning force microscopy by virtue of their low force constant (approximately 1 N/m) and high resonant frequency (more than 10 kHz). The advent of the SKPM, described by M. Nonnemacher et al., "Kelvin probe force microscopy," Appl. Phys. Lett. vol. 58, no. 25, at 2921-23 (Jun. 24, 1991), and SCM, described both by J. Matey & J. Blanc, "Scanning capacitance microscopy," J. Appl. Phys. vol. 57, no. 5, at 1437-44 (Mar. 1, 1985), and by Williams et al., "Lateral dopant profiling with 200 nm resolution by scanning capacitance microscopy," Appl. Phys. Lett. vol. 55, no. 16, at 1662-64 (Oct. 16, 1989), has placed constraints on the durability and lifetime of such tips, especially in regard to spatial resolution of electrical features. In addition, SCM, SKPM, and STM require that the tip and cantilever be highly conductive.
Conventionally, the tip of a metallic wire made of metallic atoms of nickel, platinum, tungsten, iridium, or gold was used to form the probe tip for STM. The wire was typically 1 to 0.01 nm in diameter and electrolytically or mechanically polished and machined to a sharp tip of 0.1 .mu.m in radius. See, e.g., U.S. Pat. No. 5,155,715 issued to Ueyema et al.; U.S. Pat. No. 5,509,300 issued to Chamberlin et al.; U.S. Pat. No. 5,065,103 issued to Slinkman et al.; and U.S. Pat. No. 5,357,109 issued to Kusumoto. And it is known to manufacture probes with a tip of less than 50 nm (0.05 .mu.m) in radius of curvature and a cone angle ranging from 10 to 20 degrees at the tip.
Current art is to vacuum deposit a thin film coating or layer (typically 200 .ANG.) of metal such as gold, chromium, or iridium onto a silicon tip, cantilever, or both to form the probe tip. With repeated use, however, especially when scanning over regions of high relief as occurs predominately when characterizing VLSI devices on a chip, the metal coating often flakes or wears off the tip. The metal on the tip will deform (i.e., flatten) in virtually all cases. Removal, wear, or deformation of the metal coating seriously degrades or destroys the ability of the tip to detect the desired electrical signal: capacitance, voltage, or current, respectively. Acquisition of electrical data becomes problematic given degraded sensitivity and, as the tip wears, the spatial resolution of the underlying topographic data also degrades. Reproducible data is difficult to achieve as a rule on deep submicron structures, such as VLSI devices. The conventional solution to these problems has been vacuum-deposit metalization.
Thus, although the advancing technology has addressed certain limitations in the art of scanning probe microscopy, certain disadvantages remain. To overcome the shortcomings of the prior art, the present invention is directed toward providing to the art of force microscopy an improved probe microtip able to produce high-resolution analysis of a surface, either conductive or non-conductive, at a tip distance which will not contact (through electric current or inadvertence) the surface under study. A further goal is to provide a tip which allows for measurement of both axial forces from a sample and multi-directional lateral forces, is effective in ambient conditions, is sensitive to small force gradients, and can be obtained using inexpensive laboratory instrumentation. An object of the present invention is to provide an improved tip which is more durable than previous tips and less susceptible to deleterious wear and deformation. A related object is to provide a tip which is more long-lived and stable than previous tips allowing, for example, experiments that require statistical treatment (and, therefore, data collection over an extended period of time). Still another object of the present invention is to provide a probe microtip with superior mechanical noise immunity by virtue of a higher effective force constant. Final objects are to provide a microtip which is highly conductive and to improve the electrical performance of such microtips.